A bulk material screening device is a device which separates input material by size or shape. These screening devices are used in industries such as mining and aggregates, forestry, agriculture, and recycling to separate inbound materials into more valuable products. For example, in the solid waste and recycling industry, high value corrugated cardboard containers tend to be of a larger size than other recyclable material, and so can be separated from other materials based on this large size. Traditional screening devices include trommels, disc screen, and vibratory screens.
Trommel screens and vibratory screens utilize a static screening surface, consisting of a steel or polymer material with holes of a certain size, and then bring material into contact with the screening surface such that materials smaller than the holes pass through the screening surface and material larger than the holes do not. These types of static screens are subject to operational and maintenance hazards when material builds up in or around the holes of the screening surface. This typically happens due to wet or stringy material. As material builds up around the holes, the hole size becomes smaller and the nature of the screened material changes. Once the holes become too small, it becomes necessary to stop the machine so the screening surface can be cleaned.
A disc screen consists of a series or parallel rotors or shafts, with attached discs or stars, turned in concert such that a sufficiently consistent opening between the rotors is achieved as they are turned. Screening is achieved by constructing the rotors and discs such that the desired opening is achieved. The rotation of the rotors also drives the material forward, making it easier to bring new material into contact with the screening surface, allowing for smaller and cheaper machines to be used to accomplish the same task as passive screens. This type of active screening surface is not subject to the buildup of wet materials as described above. However, the rotors are extremely prone to wrapping on stringy materials. As each rotor is increasingly wrapped, the opening of the screening surface decreases due to wrapped material. Once the openings become too small, it is necessary to clean the machine by cutting the wrappings off with a chisel or knife. The prevalence of plastic bags and other stringy material within solid waste is a consistent challenge for the waste and recycling industry when using disc screens.
It is generally understood that many of the maintenance hazards for disc screens stop being an issue as the opening of the screen gets small enough that stringy material can no longer fit through the opening. This typically occurs at an opening size of approximately two inches. However, a screen with this size opening typically cannot accept any material larger than six inches or risk having the openings covered, or blinded, by the larger material, meaning these disc screens that don't suffer maintenance issue typically are toward the end of a material processing line after larger items have been removed.
When the aforementioned machines are used in particularly difficult material stream that present high amounts of maintenance hazards, there is often a conveyor belt configured to allow human sortation of materials placed before any machines to remove said hazards. This is typically called a “pre-sort”. Pre-sorting material before mechanical process is expensive as it takes many sorters to sift through the full burden depth of the material. Further, this pre-sort station is the most hazardous sort station to the human sorters as the receive all of the heterogeneous material. They are tasked with sorting large, heavy objects from in a moving pile, which can be up to 30″ away from them, while avoiding being stabbed by broken glass, sharp metal objects, and other sharp objects such as used hypodermic needles that would typically be found in the small fraction of material. For this reason, most workers at a pre-sort station utilize Kevlar or similar gloves to protect themselves, but this makes it even more difficult to lift the intended items, requiring additional sorters and additional expense to achieve a sufficient pre-sort such that the material screening devices do not constantly break down.
Another type of active material screening device consists of a series of parallel augers with interleaved flights with consistent spacing such that the opening between auger shafts and flights creates a screening surface. An auger is a central shaft with a rotating helical blade attached to the radial surface. As a helix is necessarily a projection on the surface of a cylinder with a constant angle between the tangent of the projection and a central axis, augers are traditionally round. In the application of a screening device made out of augers, the roundness of the auger and consistent spacing of flights guarantees the adjacent augers do not collide. Auger screens are beneficial in certain industries, such as screening of solid waste materials, where wrapping and plugging of traditional screening devices is a problem. This is because as material wraps on the auger shaft, the flights of the adjacent augers pushes the wrapped material off the shaft and prevents plugging and jamming. However, as augers are constrained to a circular shape, the current state of the art in auger screens does not provide any bouncing motion or material agitation so material does not sift toward the screening surface, limiting applications to where material can be singulated or reducing screening efficiency compared to disc screens, requiring much larger machines to be used to accomplish the same task. Further, as taught by Gunther in EP 1570 919 B1, this machine is very sensitive to the material feed configuration, requiring the machine to be fed laterally with a high speed belt such that material is flung onto the machine, rather than dropped, to minimize the sorting inefficiencies of having no agitation.
It is not necessary to pre-sort an auger screen as it is with other screening devices, allowing it to be placed in front of the pre-sort. In the current state of the art, an auger screen with approximately an 8″ opening is placed before the pre-sort to screen out small and potentially hazardous items. The pre sorters can then focus on the sorting of large items which are maintenance hazards without needing to worry about being stabbed by broken glass or hypodermic needles. This further allows fewer pre sorters to be used to accomplish the same task as a traditional pre-sort. However, the auger screen is not suitable for final screening of materials, such as a typical disc screen configured for the separation of old corrugated cardboard from mixed recyclables. These machines have an approximately 12″×12″ opening with rotors on 20″ centers and an amplitude of agitation of 2″ to 5″. The high amplitude is necessary as OCC, such as the box of a flat screen television, is large and flat compared to the other items being sorted, and so other items tend to ride on top of the pieces of OCC. Further, a traditional disc screen typically requires an amplitude of at least 5% of the maximum particle size in order to achieve sufficient sifting action, with higher ratios being better. As such, an OCC Disc Screen with a 2″ amplitude would typically be used on items up to 40″ in diameter in any one dimension. With no agitation, the auger screen cannot be used for the screening of cardboard as too many riders pass over the screen.
Wess teaches of another form of auger screen in U.S. Pat. No. 9,895,719. The auger flights in this machine consist of a series of “fingers” or “stars” protruding from a substrate. While in theory this will increase the surface speed of materials on the screen deck, the distal end of the fingers describe a circle and there is too little space between the fingers to provide agitation, so it doesn't solve the primary weakness of existing auger screens. Further, this shape creates a pinching hazard as the minimum distance between the helical shaped “flight: and the opposite substrate varies continuously and sharply. This creates an impinging motion between the two mechanical parts that has a risk of causing a hard jam in the machine if a hard object, such as a rock, falls behind a finger and is forced into the opposing substrate by the following finger. While this risk is relatively small for small openings screening devices and with the fingers placed tightly together, as the opening size is increased or the fingers are moved further apart, larger and larger items can fall into the pocket created when the minimum distance is at a maximum which can then be pinched by the following finger, creating a hard jam. As such, this limits the inventions to screening of small items, which are already screenable utilizing disc screens which have inherent agitation.
What is therefore needed is a auger-type disc for use in a disc screen that overcomes these deficiencies.